MUSCLE-FIBER TYPE COMPARTMENTALIZATION AND EXPRESSION OF AN IMMATURE MYOSIN ISOFORM IN THE STERNOCLEIDOMASTOID MUSCLE OF RABBITS AND PRIMATES

The sternocleidomastoid muscle is located in the neck and is both a ne ck rotator and flexor. Cervical dystonia, a focal dystonia disorder, i s characterized by forceful involuntary contraction of a group of neck muscles, usually including the sternocleidomastoid. Little is known a bout the fiber type composition, fiber type compartmentalization and i nnervation patterns in this muscle in rabbit and primates. Sternocleid omastoid muscles from rabbit and monkey were analyzed for muscle fiber type composition and number, muscle fiber cross-sectional area and pa tterns of innervation. The sternocleidomastoid muscle was composed of two distinct regions, or compartments, with different fiber type compo sitions: an outer or superficial region composed of mostly type 2 myof ibers and an inner deep region composed of both type 2 and type 1 myof ibers. Neonatal myosin heavy chain isoform was detected in approximate ly 25% of the myofibers in both regions of the muscle. Neuromuscular j unctions were located in seven endplate bands approximately 1-3 cm apa rt throughout the length of the muscle. There is clear evidence of ana tomical subdivisions within this muscle. Not only is there variation i n fiber type composition between superficial and deep regions of the m uscle, but unlike most other mature skeletal muscles, it continues to express neonatal myosin heavy chain isoform in the adult. The motor pr ogram for neck movements is extremely complex, and the histological co mplexity plays a role in allowing for a continuum of movements of the head and neck, from maintenance of posture to rapid head movements. (C ) 1998 Elsevier Science B.V.